Before the introduction of the approach to weighing which is disclosed in the above-mentioned application, attempts to measure loads in such adverse situations as garbage handling equipment or other hostile weighing environments had followed the conventional wisdom of load cell technology, and devices have been constructed using parallelogram linkages and other complex means of isolating a load cell or cells from spurious strains. None of these approaches has succeeded in achieving sufficiently accurate and repeatable results, with durability.
The methods described in the above-mentioned International application involve a radical approach to the problem of dealing with variations in strain field orientation. In these methods, the primary sensing axis of a principal strain gauge is orientated for response to variation in a principal force, and the effect of secondary forces or secondary strain on the principal gauge is corrected by means of a secondary strain gauge located and orientated so that its response represents the influence of the secondary forces on the response of the principal gauge.
As disclosed in the above application, garbage may be weighed in this way by the application of strain gauges, to the tines of forks employed for the transport of garbage skips. The forks are mounted at their proximal ends on a supporting structure, and a gauging region is provided near the supported end of each fork. In this region a principal strain gauge is formed by a pair of strain gauge elements, one on each side of the flange of the fork, provided with their sensing grids orientated to respond to shear strain and thereby to provide an indication of the load on the fork.
A secondary gauge comprising a second pair of strain gauge elements is also provided, adjacent to the primary gauge, the grids of the secondary gauge being orientated to respond to bending strain and compression or tension strain. The first and second pairs of gauges are connected in respective Wheatstone bridges, and these bridges are interconnected with opposite polarity so that the output of the second gauges opposes that of the first, and the relative influence of the second gauges is adjusted to obtain the required compensation.